Announced retransmission random access system

ABSTRACT

A transmission system employing a satellite and a plurality of ground stations each capable of transmitting and receiving data packets to and from each other via the satellite. The satellite station comprises first logic for dividing absolute time into time frames and time frames into message (time) slots with each message slot having a small percentage thereof divided into mini-slots and with each mini-slot in each message slot corresponding to a particular message slot in a future time frame, and second logic including a random number selector responsive to the selection of message slots in the same current time frame by two or more data packets, and third logic for randomly selecting mini-slot for each message slot in which a data packet is to be transmitted in the current time frame with the selected mini-slot for each message slot in the current time frame defining the particular message slot in the future time frame in which the data packet is to be transmitted in case two stations select the same message slot in the current time frame to cause a conflict. A future frame is a time frame which occurs after the data packet in the current time frame has propagated from its originating ground station to the satellite and then back to the originating ground station.

This is a continuation of application Ser. No. 610,007, filed May 14,1984, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to a time synchronized random(contention type) access system employing a single wideband channel timeshared by a plurality of random access transmit stations in whichpackets of information are transmitted in time slots selected randomlyfrom one station to another. More particularly, the invention relates tosuch a system in which the instances of two or more transmittersselecting the same time slots for transmission of a message, resultingin a conflict, is reduced dramatically, thereby increasing theefficiency of the system, that is the useful percentage of availabletime slots.

In systems of the type being discussed there are a number of groundstations and a master station which can be either a ground station whichperforms a function of synchronizing the transmission of packets ofinformation, all of which must be retransmitted from the satellite in atime synchronous manner. Similarly, transmissions from the groundstations to the satellites are timed so that they arrive at thesatellite in a time synchronous manner.

It is apparent that without some precaution being taken that the numberof conflicts or collisions between data packets from two stationsarriving in the same time slot is an entirely random matter and occursat much higher frequency than would be the case where certainprecautions are taken, such as the techniques employed in the presentinvention. Statistically, with no precautions, the utilization of randomtime slots is approximately 37% without precaution being taken to avoidconflicts or collisions between messages from different transmitters inthe same time slots. As will be seen later herein, the efficiency of theusage of the time slots can be raised to 53% and even to 60% byemploying certain precautionary techniques which form the basis of thepresent invention, which has been given the name AnnouncedRetransmission Random Access (ARRA). The basic concept of the presentinvention is to reduce these wasteful collisions in the random accesschannel by requiring the station terminals to transmit an announcementwith each data packet or message specifying an intended retransmissiontime slot or message slot along with every transmitted message. Thus inthe event of a collision, the other terminal stations in the system willknow which slot the particular message which collided with anothermessage will be retransmitted in, and can avoid placing newtransmissions in that message time slot.

Basically the ARRA scheme set forth in the present invention completelyeliminates collision between new message transmissions and retransmittedmessages, thus resulting in a significantly higher throughput thanconventional random access systems, such as slotted ALOHA and theCapetenakis Tree algorithm. The ALOHA system is well known art asdefined in Abramson, "The ALOHA System--Another Alternative for ComputerCommunication", AFIPS Conference Proceedings, 1970 Fall Joint ComputerConference, Vol. 37, pp. 281-285. Capetanakis Tree algorithm is definedby J. I. Capetanakis, "Generalized TDMA: The Multi-Accessing TreeProtocol", IEEE, Transactions on Communications, Vol. 27, No. 10, Oct.1979, pp. 1476-1483.

The two ARRA schemes as described in detailed in the present inventionare realized with a relatively small amount of logic and storage at eachof the terminals. Two realizations are described. The first realizationis called basic ARRA and has a capacity (maximum normalized throughput)of 0.53, which means that approximately 50% of the time slots aresuccessfully utilized. The second scheme is called Extended ARRA andachieves a capacity of 0.6, at the expense of a slightly greaterterminal complexity. The main application area for the protocol ofsystems described herein are in satellite communication systems withmany relatively low cost digital terminals (ground stations) sharing acommon channel. For completely terrestrial networks with low propagationdelay, high throughput random access systems (e.g., such as Xerox'sEthernet) already exist. For the satellite channel (which has a highpropagation delay of 0.27 seconds) ARRA provides the highest throughputalong with low delay among the comparable random access schemescurrently described in the literature.

SUMMARY OF THE INVENTION

In a preferred form of the invention there is provided a transmissionsystem employing a master station such as a satellite and a plurality ofground stations capable of transmitting and receiving data packets toand from each other via said master station. The master stationcomprises first logic system for dividing absolute time into time framesand time frames into message (time) slots with each message slot havinga small percentage of its allotted time interval divided into minislotsand with each minislot in each message slot corresponding to aparticular message slot in a future time frame and second logic systemincluding random number selection means responsive to the selection ofmessage slots in the same current time frame by two or more datapackets, and a third logic system randomly selects a minislot for eachmessage slot in which a data packet is to be transmitted in the currenttime frame with the selected minislot for each message slot in thecurrent time frame defining the particular message slot in the futuretime frame in which the data packet is to be transmitted. In case twostations select the same message plot in the current time frame to causea conflict. A future frame is defined herein as a time frame whichoccurs after the data packet in the current time frame has propagatedfrom its originating ground station to the satellite and then back tothe originating ground station.

DESCRIPTION OF THE DRAWING

In the drawings:

FIG. 1 is a diagram showing the time slot arrangement for the basic ARRAsystem;

FIG. 2 is an enlarged view of one of the message slots and the group ofminislots that accompanies each message slot of FIG. 1, and also shows ablow up of the minislots that accompanies each message slot;

FIG. 3 is a diagram of the message slots contained in a frame of theextended ARRA system and also the group of minislots in the commonminislot pool (CMP);

FIG. 4 is an expanded view of one of the message slots of FIG. 3 andalso an expanded view of the minislots associated with that particularmessage slot;

FIG. 5 is an expanded view of the minislots in the common minislot poolof FIG. 3 employed in the extended ARRA. It will be noted that the basicARRA shown in FIGS. 1 and 2 has no common minislot pool;

FIG. 6 is a general block diagram for implementing channel format forthe basic ARRA shown in FIGS. 1 and 2 and also for implementing theextended ARRA channel format shown in FIGS. 3, 4 and 5 with thedifference for implementing the basic and extended ARRA formats beingprimarily in the logic contained in the transmit scheduler 210 of FIG.6;

FIG. 7 is a block diagram of a transmit scheduler employed to implementthe basic ARRA format shown in FIGS. 1 and 2;

FIG. 8 is the schematic diagram for implementing the extended ARRAformat with of the transmit schedule for implementing the extended ARRAformat shown in FIGS. 3, 4, 5.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 there are shown two frames of the basic ARRAsystem, 98 and 99, each having a number of message slots, numbered from1 to K. Each frame is t_(f) seconds in length and each message slot hasa time duration of t_(m). Two of the message slots in frame 99 areidentified as message slots 100 and 101. Also shown are a group ofminislots whose time duration is very small compared to that of amessage slot and occur at the beginning of each message slot. Formessage slots 100 and 101 this group of minislots, which can number fromtwo up to perhaps a dozen, depending on the size of the system, arelabeled by reference characters 104 and 106 and cover a total timeperiod which is less than 1% of the time duration of a message slot andis more likely to be less than 0.5% of the time duration of the singlemessage slot.

For the purposes of discussion of the specification, assume that thenumber of minislots at the beginning of each message slot in FIG. 1 is8, so that in FIG. 2, which shows an expanded view of the minislot, K isequal to 8 and Δt is the time duration of a single minislot. Thus K.Δtcan be of the order of 0.5% of t, which is the time duration of themessage slot. It will be noted that t_(m) is a time period (as shown inFIG. 2) equal to the time period t of the message slot minus KΔt or thetotal time period of the K minislots.

In both the Basic and the Extended ARRA formats there are two types ofdata packet transmission. There is an original transmission which is atransmission that occurs for the first time, and a retransmission whichis a previously unsuccessful transmission that occurs for a second timeor perhaps a third time, as will be seen later. Assume that in thesystem being discussed there are ten ground stations identified asstations A, B, C, D, E, F, G, H, I and J, each of which is capable ofgenerating and receiving packets of data.

As discussed above, there are two types of formats that can be employedto cover two types of collisions between the data packets in a givenmessage slot. The first case, which employs the channel format for basicARRA, is the simplest format and processes those instances where twostations transmit at the same time in the same message slot.

Assume the simplest situation where no conflict can occur. Assume thatstation A wants to send a message to station B. Station A generates apacket of data and by random selection of message slot in the very nextframe transmits it via message slot 94, in frame 98 to station B.Message slot 94 contains 8 minislots as defined above. When Station Agenerates this packet of information, one of the minislots in messageslot 94 is picked at random and a pulse placed therein. Let's assumethat this minislot is minislot 2 in FIG. 2. If Station A and Station Bare the only two stations involved and no other stations are sendingmessages, there are no complications. The message is transmitted via thesatellite, Station B receives it, and there is a portion of the datapacket which contains an address which identifies Station B as therecipient of the message. Station B recognizes the message as being itsown and receives and stores it. However, it should be noted that allstations in addition to Station B and A receive the information that amessage has been transmitted in message slot 94 and that minislot 2 hasbeen selected. The selection of message slot 94 means that if anotherstation had in fact transmitted a message such as Station C during thesame message slot 94, the information contained in the message slotwould be garbled and unintelligible to both recipients.

It has been assumed that minislot 2 in message slot 94 had been pickedto accompany the message transmitted from Station A. Now the selectionof minislot 2 indicates that in case of a conflict, such as will nowoccur between Stations A and C in message slot 94, a retransmission mustoccur during the next available frame in which a retransmission canoccur. Such retransmission by station A will be to that message slotidentified by the pulse in minislot 2, which will correspond to messageslot 2 in the next frame in which the retransmission can occur.Correspondingly, if Station C placed a pulse in minislot number 3, itwould retransmit in message slot 3 of the future retransmission frame.

It should be noted that the next frame in which retransmission can occuris defined as follows. A message transmitted from Station A, forexample, must radiate to the satellite and then be retransmitted back tothe ground station, which requires a finite amount of time. The firstcomplete frame that is received upon completion of this round trip ofthe message originating at Station A is examined by logic in each of theground stations to determine which minislots have pulses therein. Allthe stations will know that Stations will retransmit in the announcedmessage slots 2 and 3 respectively. It should be further noted thatretransmission of the data packets from Station A and C occur in theimmediately occurring frame after the first frame received after theround trip interval defined above, so that the retransmission of thedata packets of Stations A and C will occur after a second round tripinterval of the data packets to the satellite and back to the groundstation. However, all other stations except Stations A and C will beable to transmit new packets of information, in all message slots in theretransmission frame excluding those indicated by minislots accompaniedby collisions in the message slot. This is further explained in thefollowing.

Let's assume Station A wants to send a message to Station B. It getsthis packet of information and by random selection sends it in messageslot 94. This message slot 94 contains a number of minislots which havebeen labeled 1, 2, 3 . . . K in FIG. 2. When Station A generates itsdata packet it's going to put a pulse at random in one of the minislots.Assume it picks minislot 2 and places a pulse in minislot 2. If StationA and Station B are the only two stations involved, and no other stationis sending messages, there are no complications. The message istransmitted via the satellite, Station B receives it, and there is apart of the packet of information that contains an address to Station Bwhich says, in effect, Station B this is your message. Station Brecognizes the message as being its own and receives and processes suchmessage. However, it's very possible that one of the other stations, forexample Station C, transmits a message to Station D about the same timeStation A transmitted its message. Station C formed its package andtransmitted it during message slot 94. Now there are two messages in onemessage slot 94 and they are conflicting. One of them is addressed to goto Station B and the other is addressed to go to Station D. Assume thatStation C arbitrarily picked at random minislot 3 as its retransmissionchoice in case of a conflict. The selection of minislot 3 has thefollowing meaning. There are two stations, A and C, transmitting in thesame message slot 94. Station A, by its selection of 2, had declaredthat it will retransmit its packet in the event of a conflict in thelater message slot 2 during a subsequent frame, which frame is measuredby the round trip propagation time of the entire system from one groundstation to the common satellite and back to another ground station.Station C by its selection of minislot 3 has declared that if there is aconflict it will retransmit its message in message slot 3 on the nextframe after the round trip propagation time of the whole system whichwould be the same frame as the retransmission of the message originallytransmitted by Station A.

The information regarding the minislots 2 and 3 of Stations A and C isretransmitted via the satellite to all of the ground stations in thewhole system so that all the ground stations know there is going to be aretransmission on the next available retransmission frame in minislots 2and 3. Such other stations, however, do not know that thesetransmissions are going to be from Stations A and C and they further donot know the destinations of the retransmissions. The other stationssimply know that these two message slots 2 and 3 in the nextretransmission frame after the first round trip propagation time of thewhole system are going to be occupied. Consequently, the remainingstations will consider the message slots 2 and 3 in this second frame,the retransmission frame, off limits to them for that frame. It isobvious that the system has now reduced the total number of possibleconflicts and increased the efficiency of the system.

Assume now that the next frame does occur and that in message slot 2there will be a retransmission from Station A, which will be addressedto Station B and received by Station B. The next message slot in thesame frame will be message slot 3 and the recipient Station D which willreceive and process such message. However, other complications can arisebecause somewhere in the system there could have been another station,such as Stations F and G, sending messages at the same time. That is tosay that Station F transmitted a message and Station G also transmitteda message in the first frame 96 and selected by random selectionminislots 2 and 4, respectively. More specifically, assume that StationF selected minislot 2, and Station G selected minislot 4. Stations F andA have now both selected minislot 2 in the next possible retransmissionframe occurring after the round trip propagation time to the satelliteand back to the ground station.

When retransmission of the data packets of transmitters A and F isattempted in this first retransmission frame after the first propagationtime interval, it is apparent that the system is retransmitting twomessages from A and F in the same message slot, namely, message slot 2in the retransmission frame after the first propagation time interval.It is also apparent that the two messages originating from transmittersA and F, although they are retransmissions, represent essentially thesame problem as the original conflicting transmission from transmittersA and B. Thus, when transmitters A and F attempt to transmit in messageslot 2 in the second frame, each will pick a new minislot at random.Assume that this time transmitter A selects minislot 6 and F selectsminislot 8. When they retransmit after the second propagation timeinterval of the system it is possible that there will be no conflict.

As discussed briefly above, there is a structure in FIGS. 6 and 7 toimplement basic ARRA format shown in FIGS. 1 and 2. The structure ofFIG. 7 and the structure of FIG. 8 is employed to implement the extendedARRA format shown in FIGS. 3, 4 and 5.

Before discussing the structure of FIGS. 6, 7 and 8, consider first thechannel format for the extended ARRA shown in FIGS. 3, 4 and 5, which isan improvement over the basic ARRA of FIGS. 1 and 2. As background, itwill be recalled that in FIGS. 1 and 2 it's possible to have twostations A and C as transmitters which select the same minislot randomlyso that when the retransmission frame occurs the messages sent bytransmitters A and C cannot be successfully transmitted to theirdestination because the conflict has already been established byselection of the same minislot 2. This is a problem that is solved bythe structure of FIGS. 3, 4 and 5, which uses a common minislot pool(CMP). In the logic implementing the extended ARRA system shown in FIGS.3-5, there is additional logic in each of the ground stations which willidentify the situation described immediately above where Stations A andC selected minislots 2 and will instruct terminals A and C to aborttheir announced transmissions and will remove from the RAM in all groundstations the fact that message slot 2 defined by minislot 2 isunavailable, which then in fact makes minislot 2 in the retransmissionframe available to any station except Stations A and C. The logic alsomust reschedule the transmission from A and C to some other message slotin the future frame beyond the first retransmission frame after thefirst round trip propagation transmission to the satellite. This is doneby making a corresponding announcement by selecting any of the CMPminislots whenever a retransmission is aborted.

Logic is provided in the ground station that will select a new minislotfor each of transmitters A and C. Assume that transmitter A picksminislot 1 (in the CMP) and transmitter C picks minislot 5 (in the CMP)after the second propagation trip to the satellite. In the next frame,after the next propagation time to the satellite, transmitters A and Cwill transmit their message in message slots 1 and 5 provided there isno conflict with other retransmitted messages. If there are conflictswith other retransmitted messages, the system must go through thisprocess again.

It is important to note, however, that in extended ARRA when a conflictoccurs, the system does not actually make a retransmission. If there isa conflict the system avoids the transmission and reschedules it untilthe next frame or the next frame after the next frame, etc., until allconflicts are resolved.

At this point the general discussion of FIGS. 1-5 has been substantiallycompleted, and a discussion of the hardware to implement the format ofFIGS. 1-5 will be discussed.

Consider first the block diagram of FIG. 6, which is generic to both thebasic ARRA format shown in FIGS. 1 and 2 and the extended ARRA formatshown in FIGS. 3, 4 and 5.

Referring now to FIG. 6, data in the form of data packets is suppliedfrom data source 199 and delivered via leads 200 to an input data sourcebuffer 202, which can be a rather large first-in, first-out (FIFO) databuffer. As indicated, the data is divided into packets so the FIFOitself, although standard, is fairly large and can be fairly complex.The data packets are extracted from FIFO 202, one by one via the cable204 to a single packet transmit buffer 206, which stores the data packetuntil the proper timing pulse on lead 256 passes them via the multi-leadbus 208 into transmit scheduler 210. The transmit scheduler 210 is acomplex piece of equipment, and serves both the basic ARRA format ofFIGS. 1 and 2 and the extended ARRA format of FIGS. 3, 4 and 5 in amanner shown in FIGS. 7 and 8, respectively.

The transmit scheduler 210 performs three major functions as set forthbelow.

1. It assigns message slots for new transmissions. This requires thatthe transmit scheduler contain a table in the form of a random accessmemory (RAM) to maintain a record of message slots that have announcedretransmissions in which it has already been announced via minislotsthat messages are to be transmitted. The RAM table must contain alisting of all the message slots which cannot be used in the next frame.

2. The transmit scheduler selects minislots which must accompany eachmessage transmission. Thus, in case a retransmission is required, it isnecessary to choose new minislots which are selected by a random numbergenerator. The transmit scheduler must therefore contain a random numbergenerator to select new minislots.

3. A third primary function of transmit scheduler 210 is to delay andretransmit colliding messages as announced by the station in question.The foregoing requires selectable delays to place the data packet in theannounced message slot location. This means that the data packet whichis going to be retransmitted must go in the message slot selected by therandomly selected minislot in the next frame, and identifies thespecific message slot in which the packet will be stored andretransmitted.

The output of transmit scheduler 210 is a data packet supplied via bus212 which can be either a serial or parallel transmission means to thefull duplex modem 214, then via bus 216 to the up converter 218. Theoutput of modem 214 is supplied serially, bit by bit, to the upconverter system 218, then to an RF amplifier 220 to antenna 222, thento the satellite, not shown in FIG. 6.

The transponder in the satellite reflects the received data back whereit is picked up by an antenna corresponding to antenna 222 of each ofthe ground stations, then through amplifiers corresponding to amplifier220 of FIG. 6, to the RF subsystem which is now a down converter, thenin parallel to bus 216 into full duplex modem 214. The received data isthen supplied in parallel form via bus 226 to the cyclic redundancycheck (CRC) decoder 228, which determines if there is an error. If anerror exists it will almost always be due to more than one message beingreceived at a given time. How such an error is detected is a separatesubject and is explained in detail in the following publications.

"Technical Aspects of Data Communications", J. E. McNamara, DigitalPress, Bedford, MA, 1982 (Chapter 13, pp. 110-122).

"Errors and Error Control", H. 0. Burton and D. D. Sullivan, Proceedingsof IEEE, Vol. 60, pp. 1293-1301, Nov. 1972.

The conclusion of CRC 228 as to whether an error exists is a go or no godecision. If the decision is that no error is present and the message iscorrect, such message is supplied to the address recognition circuit 232via lead 230, then via bus 234 in parallel to a data receiver 235 whereit is utilized.

The foregoing is an example of a perfect transmission where no problemsexist, that is, no conflicts or collisions have occurred.

Assume now that a conflict or collision does occur. More specificallyassume that two channels or two transmitters A and B both transmit atthe same time on the same message slot. It is to be understood that theconflict occurs upon the reception of the data packet from two differenttransmitters since the transmit scheduler 210 is unique only to the datapacket generated in the single transmitter shown in FIG. 6. Thus theoutput of transmit scheduler 210 initially is supplied to full duplexmodem, which is also unique to only one transmitter at this point. Theoutput of modem 214 goes to converter 218 and antenna 222 as a cleansignal. If transmitter C had transmitted at that same time, thesatellite would be retransmitting back at the same time, and transmitterA of FIG. 6 would receive two messages, one originating from transmitterA and the other from transmitter C. Both data packets are being receivedby antenna 222 at the same time and being supplied to down converter 218and into full duplex modem 214 simultaneously. One of these signals willbe a signal via dotted lead 224 from full duplex modem 214 to transmitscheduler 210 and labeled "received minislot announcement" which is avalid announcement to be acted upon only if the message error indicator252 is positive (error condition). The system always receives thatsignal back because it always has at least one minislot filledindicating the proper message transmitted. However, the system will alsoreceive an output from CRC 228 indicating an error via lead 252, whichis supplied via lead 258 back to transmit scheduler 210, which indicatesthere is an error and that error is probably the fact that twotransmitters were transmitting at the same time. The transmit scheduler210 will then examine the output of lead 252 and conclude that an errorexists and that certain results must occur. These results are thoseshown essentially to the right of transmitter scheduler 210 in FIG. 6and set forth above in the specification.

If the system had a collision of two messages in message slot 2 asreceived by antenna 222, then the random selector in the transmitscheduler 210 of FIG. 6 must schedule retransmission as announced. Ifminislot 2 has been selected, then the data package will beretransmitted in message slot 2 in the next frame.

If there is no conflict or collision, a signal appears on lead 256 fromtransmit scheduler 210 to buffer 206, which indicates that thetransmission was all right, without collision with another transmission,and a signal is sent via lead 260 to FIFO 202 indicating that anotherdata packet can be accepted by single packet transmit buffer 206 foranother transmission.

Consider now the block diagram of FIG. 7 which is basically an expansionof the transmit scheduler 210 of FIG. 6. A received minislotannouncement is supplied via lead 224 from modem 214 of FIG. 6 to table300 of FIG. 7 and a message error indicator is supplied via lead 252from the CRC logic 228 of FIG. 6 to block 300 (via lead 252) of FIG. 7.As discussed in connection with FIG. 6, if a one appears in both inputleads 224 and 252 into logic 300, it indicates a collision, that is anerror, and requires retransmission of the colliding data packets. Aminislot defined by the pulse supplied via lead 224 is dumped into thetable 300 and ultimately, as will be explained later, the sametransmission packet is accessed in the single packet transmit buffer 206and retransmitted.

It should be noted that the logic 300 contains the unavailable messageslots in the next frame. At the end of the current frame, before theoccurrence of the next available frame, the identification of theunavailable message slots are dumped into a RAM table contained in block302. The framing sync signals, the message sync signals, and theminislot sync signals appear on leads 242, 244 and 246 in FIG. 6 and areall generated by timing sync module 240 which is common to all of thetransmitters. The timing sync module 240 in turn derives its timingsignal from the output of the full duplex modem 214 of FIG. 6 in a wellknown manner. When each new packet of information arrives in a singlepacket transmit buffer 206 of FIG. 7 from FIFO 202 of FIG. 6 thetransmission of that new packet is initiated. While the process is infact started when the new packet arrives in the single packet transmitbuffer 206 of FIG. 7, it does not actively enter into the process ofselecting a message slot until the start of the next frame. Such delayexists until the beginning of the next frame when one of the availablemessage slots is selected by using the information of table 302. Asblock 302 contains the table of unavailable slots, the logic in block310 must in fact select one of the remaining message slots to obtain anavailable and usable message slot. A random selection is effected bymeans of a random number generating logic contained in the logic ofblock 310. Assume that the message slot randomly selected is messageslot 7. Appropriate timing and delay means is provided within block 312to delay the placing of the data package into the frame until messageslot 7 occurs, at which time the data package is inserted into messageslot 7 in block 312.

It is now necessary to randomly select one of K minislots forretransmission announcement purposes, as discussed above, in the eventof a collision. Such random selection of a minislot is done within block314 which must contain random number generator logic and be accompaniedby a record keeping means, such as a RAM, (in block 320), to recordwhich particular minislot has been selected to correspond to theselected message slot 7. Assume, for purposes of discussion, thatminislot 3 has been selected. Value 3 is placed in the RAM in block 320,and a pulse is placed in word location 3 of the RAM data buffer in block316 to indicate selection of minislot 3.

In block 320 the randomly selected minislot is stored for laterreference. Also, the selected minislot is appended to the data packet inlogic block 316 to assume the format shown in FIGS. 1 and 2 to form thecomplete packet which is then supplied via lead 212 to the modem 214 andthen the up converter RF, amplifier 220 and antenna 222 fortransmission.

Upon retransmission from the satellite, the transmitter of FIG. 6, nowfunctioning as a receiver, receives the signal that's transmitted andsupplies it to amplifier 220, down converter 218, full duplex modem 214,CRC logic 228 of FIG. 6, and if the message is correct, through logic232 to a data receiver 235. The logic blocks 320, 322 and 324 correspondgenerally to the logic 228, 232 and 235 of FIG. 6. More specifically, ifdecision block 324 of FIG. 7 after observing the signal on lead 252finds no error then it energizes the clear buffer signal logic 326 toclear the single packet transmit buffer logic 206 which then applies anew data packet into the system via delay means 308. If there is anerror, that is a collision resulting from the simultaneous transmissionof two data packets, then the output of decision block 324 so indicatesby sending a signal to logic 330 which is labeled "take packet frominput buffer." The input buffer so indicated is the FIFO 202 shown inFIG. 6 and in fact means the retransmission of the same data packetwhose transmission has failed.

In FIG. 7, if two new transmissions decided to use message slot 4 andone transmission arbitrarily selected minislot 1 and the other selectedminislot 2, there would of course be a conflict in message slot 4. Butin the retransmission frame, one of the packets would be transmitted inmessage slot 1 and the other in message slot 2, and the conflict wouldbe avoided assuming that no other message packets from othertransmitters were involved. Other new transmissions could pick minislots3, 4, 5, 6, 7, 8 in said retransmission frame.

Alternatively, the system in FIG. 7 could have three messages allpicking message slot 4 with one of the message slots picking minislot 4and the other two message slots picking minislots 2 and 3 so the onlyminislots left are 4, 5, 6, 7, 8. Any new transmissions would have topick one of these remaining minislots.

It should be noted that if there is a retransmission occurring at anystation that station cannot accept a new transmission at that time. Thestation, such as the station shown in FIG. 7, must wait until allretransmissions are resolved. Then the station of FIG. 7 can generate anew transmission. However, it is possible to have the new transmissionfrom the transmitter of FIG. 7 even though another transmitter, such astransmitter B, is currently working with an old transmission orretransmission.

Returning again to a discussion of the station of FIG. 7 receiving asignal back from the satellite which contains two or more simultaneousdata packets and therefore an erroneous signal. Such erroneous signal isdetected in block 324, indicated to block 330, which takes the same datapacket from the input buffer 260 of FIG. 6, as discussed before, forretransmission in the next available frame. The logic of block 330 alsosupplies a signal to block 332 labeled as a delay to the message slot M(the value of which was stored in block 320) of the next frame. Thismessage slot is then supplied via lead 336 to block 314 in which anotherminislot is selected randomly and handled by the logic of block 316 inthe manner described above. In block 318 the data packet is transferredto the modem 214 for subsequent retransmission via antenna 222 in themanner described hereinbefore. Upon reception of the message back fromthe satellite it flows through modem 214, CRC logic 228 and logic 232,if there is no error. If there is an error then the signal is processeddown to transmit scheduler block 210 as described above, and the wholeprocess is repeated again.

Turning now to the logig of FIG. 8, there will be set forth a fuller andmore detailed description of the operation of this Extended ARRA mode ofoperation, the concept of which is generally as follows, and is, ineffect, a system which processes second order conflicts or collisions,thereby improving the efficiency further.

A general description of extended ARRA follows.

An examination of the basic ARRA protocol described above reveals thatdue to the fact that announced retransmissions take placeunconditionally, a number of guaranteed collision situations amongretransmitted messages occur. To improve the capacity, theretransmission strategy of Basic ARRA can be slightly modified to avoidsuch predictable collisions. In particular, it is clear in basic ARRAthat if two or more unsuccessful messages in a frame select the sameretransmission slot in the next frame, a collision will certainly occur.Consider first an idealized scenario in which ternary feedback (empty,success i.e. one transmission, collision i.e. two or more transmissionsis also available from the announcement minislots. Note that this mayimply the transmission of more than one bit per minislot to bepracticable. Also consider an extension to the channel format of BasicARRA, as shown in FIG. 2, in which each frame begins with a group of Kminislots which is not associated with any of the K message slots. Thisgroup of minislots is referred to as the common minislot pool (CMP). Ina system with T second message packets, the proportional overheadintroduced by this format is of the order of (K+ 1).sup.Δ T/T (see FIGS.3A and 5).

As with Basic ARRA protocol, users monitor the active minislotsassociated with a message collision (C). In addition, all transmissionsin the common minislot pool are also noted, as if they were accompaniedby a status=C message slot. Now instead of merely noting down the activeminislot indices, a pair of values (m_(i) =index, v_(i) =status and canrepresent either a successful transmission (S), a collision (C), or anempty mini-slot (E) is saved for each such minislot. The value of v_(i)for ternary feedback (the 3 conditions S, C or E) may be either S or Cfor an active minislot. In Extended ARRA, the goal is to identifyconflicts among announced retransmissions and abort these, therebyfreeing up a larger portion of the frame for use by new transmissions.Aborted retransmissions obviously require a mechanism for announcing theintended slot for the next retransmission attempt; hence the use of thecommon minislot pool. Whenever a retransmission is aborted, thecorresponding retransmission announcement is made in the common minislotpool.

To identify conflicts, first all entries of the type (m_(i), C) areidentified and the corresponding retransmissions are aborted. Thisprevents two colliding messages from retransmitting in the same slot intheir next trial. Next, all retransmissions corresponding to entrieswhich occur more than once are identified and aborted. This stepprevents unsuccessful messages in different slots of the present framefrom selecting the same retransmission slot in the next frame. Finallyonly those (m_(i), S) entries which occur exactly once are identified asnon-conflicting and retransmissions corresponding to these are allowedto take place. Correspondingly, the disallowed set D for newtransmissions is formed by taking the union of these unique entryindices. Observe that these steps guarantee that there will be nocolliding retransmissions i.e. those which have not been aborted will besuccessful. To summarize:

Extended ARRA

1. All transmissions are accompanied by a retransmission slot (randomlyselected from 1 to K) announcement in the minislotted subchannel.

2. In frame J, new transmissions originating in frame (J-1) aretransmitted in a randomly selected message slot from the allowed setA=F-D.

3. The set D for frame J is computed by monitoring transmissionsreceived during frame (J-1) in the following manner:

(i) Active minislots with a collision in their corresponding messageslot and those in the common minislot pool are noted, and a pair (m_(i)=index, v_(i) =status) is saved.

(ii) Retransmissions associated with entries having status=C areaborted. Then retransmissions associated with repeated (two or moresuccessful announcements of the same index) entries with status=S areaborted.

(iii) All aborted retransmissions are replaced by an announcement forthe next retransmission attempt in the common minislot pool of the nextframe.

(iv) The disallowed set D is computed as the union of the unique slotindices corresponding to entries which remain after step (ii).

4. Only the retransmissions associated with unique (m_(i), S) entries(identified in 3-(iv) (above) are actually placed on the announcedmessage slots of the next (Jth) frame.

Supposing transmitters A and B both want to transmit a message onmessage slot 4. A selects a minislot 1, and B selects minislot 2. Thereis no conflict at this point. However, if there is another pair oftransmitters in say message slot 7 of the current frame, for exampletransmitter C, involved which selects minislot 2 and a fourthtransmitter D which selects minislot 5, then there are two transmittersB and C selecting message slot 2 in the retransmission frame which willcreate a conflict. This is the conflict that the structure of FIG. 8will avoid.

The system requires a common minislot pool shown in FIGS. 3, 4 and 5.The specific problem is that channels B and C have both selected messageslot 2 in the retransmission frame. All of the other transmitters knowthis by observing minislots, have stored it in their RAM memories, thatis the tables which are formed in their RAM memories. The system logicis arranged so that neither transmitter B nor transmitter C can actuallyuse the conflicted message slot 2 in the retransmission frame. However,because transmissions from B and C are aborted, any other transmittercan use message slot 2 in this same frame for new messages. For example,transmitter H can use message slot 2. Transmitter H will know from theadditional logic incorporated here that message slot 2 is availableduring the current frame. This is in contrast to Basic ARRA in whichmessage slot 2 is not available to transmitter H and a wastefulcollision between B and C occurs in slot 2. B and C must have logic thatwill indicate to transmitters B and C when to abort announcedretransmissions for which conflicts can be predicted. All three of thetransmitters, B, C and H are looking at their own memories and are goingto interpret them differently.

Thus, whenever it appears that two retransmissions are going to occur inmessage slot 2 in the next frame, it is identified as a predictableconflict. So the question arises as to why let such a conflict occur atall. As described above, The conflict is avoided by simply barring bothtransmitters B and C from transmitting in message slot 2 in the nextframe. Because it is not used by B and C, message slot 2 in the nextframe can accept messages from transmitters other than B and C. Also,since retransmission from B and C have been aborted, a way ofrescheduling their retransmissions is needed. This is done simply byplacing a pulse at random in one of the K common pool minislots (CMP),indicating the selected message slot in the next retransmission frame.The system works by adding some logic to the interpretation of the tableof unavailable slots recorded by each terminal. Any terminal that has inits RAM memory the number 2 (representing a minislot) occurring two ormore times indicates that any transmitter can take the message slotrepresented by minislot 2, except stations which announcedretransmissions for the conflicting slots in question.

The question arises as to how the other transmitters know that theappearance of the numeral 2 twice in the minislot represent transmittersA and C. This information is contained in the repeated minislot entrytable in block 446 of FIG. 8. Examination of this table preventstransmitters A and B from using message slot 2 in the next occurringframe.

Consider now the upper right hand portion of FIG. 8. A minislotannouncement via lead 448 is received from modem 214 of FIG. 6 which issupplied into a word location in RAM 444 which is a table of unavailableslots in the next frame, that is, slots unavailable to new transmission.Logic block 444 not only notes the presence of a 1 in a minislot and amessage error, but also notes the common minislot pool announcement oninput lead 448.

The contents of table 444 are transferred once every frame to block 400,repeated minislot entries are eliminated from the table of unavailableslots (which is a function of the Extended ARRA logic of FIG. 8 and isnot obtainable with the logic of FIG. 7).

The elimination of repeated minislot entries from the table ofunavailable slots in block 400 and the transfer of the remaining slotsinto block 402 leaves in block 402 a table of available slots to newtransmissions in the current frame. Simultaneously, the list ofeliminated minislot entries is stored in block 446. A delay is built inblock 452 until the start of the next frame, at which time a new datapacket is supplied from the single packet transmit buffer 454 into thesystem to block 452 and then at the proper time into block 404. Thedelay 454 is of sufficient duration to insure that the selected messageslot which we'll assume to be J noted in block 404 is in a time positionto receive the new data packet stored in the delay means 452.

In block 406 the data packet goes to block 408 where one of the Kminislots is randomly selected and then appended to the data packet inblock 410. The appending of the minislots of the data packet means thatthe minislot is positioned in a small dedicated section of the RAM inblock 410 of FIG. 8. Subsequently, in block 412, the data message istransferred to the modem 214 of FIG. 6 for transmission via antenna 222to the satellite and then to the other stations. The announced minislotnumber 404 in passing through blocks 406, 408, 410 and 412 is suppliedto storage logic 414 where the announced minislot number is stored. Whenthe transmitted data packets arrive back from the satellite thetransmitter of FIG. 8 compares it with the transmitted message receivedto determine whether an error occurred. The decision as to whetherconflict occurred is made in decision block 418 by examining lead 450.If there is no error then the signal is sent to buffer signal logic 420which asks the single packet transmit buffer 454 for a new data packetfor the next transmission and the process starts anew. If there is anerror detected in block 18, then the retransmission cycle is started.First, the block 424 verifies whether the announced retransmission wasfree of conflict. This is done by comparing the announced retransmissionminislot (stored in 414) with the list of repeated entries in block 446.If no conflict exists, the retransmission is allowed to proceed asannounced via blocks 442, 426, 408, 410 and 412, as in previouslydescribed cases. If there is a second order conflict (detected in block424), then the announced retransmission is aborted by setting null datain block 480 and then transmitting only a minislot announcement in theCMP via blocks 438, 408, 410 and 412.

Reference is made to co-pending application S. N. by DipankarRaychaudhuri, filed June 11, 1982 and entitled "Time Division MultipleAccess Communications Systems," and incorporated herein reference foradditional description of some of the principles of time slots and theiruse by data packets.

What is claimed is:
 1. A contention system for transmitting information among a plurality of transmitter-receivers along a transmission path having a delay which is long compared with the duration of an information message, so that the transmitter-receivers cannot directly establish the current status of the transmission path, the system comprising:means for establishing a uniform time frame among said transmitter-receivers, said time frame including recurrent frame intervals, each frame interval including a predetermined plurality of sequential information message intervals, each information message interval including a minislot interval preceding an information transmission interval, each minislot interval including a plurality of sequential minislots equal to said predetermined plurality; random initial transmission time selection means associated with each of said transmitter-receivers for accepting during a current frame interval information to be transmitted and for randomly selecting as a selected information message interval during which transmission will occur one of said information message intervals from among available ones of said predetermined plurality of information message intervals in the selected frame interval next following said current frame interval; random retransmission time selection means associated with each of said transmitter-receivers for randomly selecting as a retransmission information message interval one of said information message intervals from among said predetermined plurality of information message intervals in a retransmission frame interval following said selected frame interval by at least said delay; announcing and transmission means associated with each of said transmitter-receivers for transmitting an announcement message during the minislot interval of said selected information message interval, and within said minislot interval of said selected information message interval, within that minislot corresponding in minislot sequence within said minislot interval to the sequential position of said one of said retransmission information message intervals within said retransmission frame interval, thereby announcing to all receivers that particular information transmission interval during which retransmission of said information to be transmitted will occur in the event that a collision occurs in said selected information message interval within said selected frame interval; collision identifying means associated with each of said transmitter-receivers for identifying collisions within said information message intervals; identifying and inhibiting means associated with each said transmitter-receivers for identifying said available ones of said predetermined plurality of information message intervals in said selected frame interval, said identifying and inhibiting means excluding as available information message intervals those information message intervals which are both announced by any of said announcing and transmission means as being for retransmission of information in the event of a collision, and identified by said collision identifying means as being associated with a collision.
 2. A contention access communications system for transmitting information among a plurality of transmitter-receivers by way of a transmission path having a time delay between any transmitter and any receiver which is long by comparison with the duration of an information message, the system comprising:timing means for establishing a uniform time frame among said transmitter-receivers, said time frame including recurrent frame intervals, each frame interval including a common minislot interval and an information interval including a predetermined plurality of message slots, said common minislot interval including a plurality equal to said predetermined plurality of time-sequential common minislots, said message slots each including an information message interval and an individual minislot interval including a plurality equal to said predetermined plurality of individual minislots; random initial transmission time selection means associated with each of said transmitter-receivers for accepting during a current frame interval information to be transmitted and for selecting as an initial transmission message slot one of a plurality of available ones of said predetermined plurality of message slots in an initial transmission frame interval following said current frame interval; random retransmission time selection means associated with each of said transmitter-receivers for randomly selecting as a first retransmission message slot one of said message slots in a first retransmission frame following said initial transmission frame by at least said delay; announcing and transmission means associated with each of said transmitter-receivers and responsive to said timing means, to said random initial transmission time selection means, and to said random retransmission time selection means for transmitting said information during said initial transmission message slot, and for, during said individual minislot interval of said initial transmission message slot, transmitting a first retransmission announcement in that individual minislot corresponding in individual minislot sequence within said individual minislot interval to the sequence position of said first retransmission message slot within the sequence of said message slots of said first retransmission frame, thereby announcing to all said transmitter-receivers that particular message slot in which retransmission of said information is scheduled to take place in the event of a collision occurring in said initial transmission message slot; collision identification means associated with each of said transmitter-receivers and responsive to said announcing and transmission means for identifying a collision within said information message interval of said initial transmission message slot; counting means associated with each of said transmitter-receivers and responsive to said collision means and to said announcing and transmission means for establishing the number of collisions which occur in each frame for which each retransmission message slot is selected, and for identifying said first retransmission message slot as one of having been selected once in said first retransmission frame and therefore being a valid first transmission slot and having been selected more than once in said first retransmission frame and therefore being an invalid first retransmission slot; first inhibiting means coupled to said counting means and to said random initial transmission time selection means for excluding from said plurality of available message slots those message slots of a frame interval designated as valid; second random retransmission time selection means associated with each of said transmitter-receivers and coupled to said counting means, for, in response to designation of said first retransmission message slot as invalid, randomly selecting a further retransmission message slot from among message slots of a further retransmission frame following said first retransmission frame by at least said delay; and first retransmission means coupled to said random retransmission time selection means, to said collision identification means and to said counting means for retransmitting said information during said first retransmission message slot if said counting means designates it as valid, and, when said counting means designates said first retransmission message slot as invalid, for inhibiting retransmission of said information during said first retransmission frame, and for, during said common minislot interval of said first retransmission frame, transmitting a further retransmission announcement in that common minislot corresponding in common minislot sequence to the sequence position of said further retransmission message slot within the sequence of said message slots of said further retransmission frame, thereby announcing to all said transmitter-receivers that particular message slot in which further retransmission of said information is scheduled to take place.
 3. A system according to claim 2 whereinsaid collision identification means further identifies collisions occurring within said individual minislot interval of said initial transmission message slot and within said common minislot interval of said initial transmission frame interval.
 4. A system according to claim 3 wherein more than one bit per minislot is transmitted.
 5. A contention transmitter-receiver for a contention system for transmitting information along a transmission path having a delay which is long compared with the duration of an information message, so that the transmitter-receiver cannot directly establish the current status of the transmission path, the system including means for establishing a uniform time frame along all associated transmitter-receivers, said time frame including recurrent frame intervals, each frame interval including a predetermined plurality of sequential information message intervals, each information message interval including a minislot interval preceding an information transmission interval, each minislot interval including a plurality of sequential minislots equal to said predetermined plurality;said transmitter-receiver comprising: random initial transmission time selection means for responding during a current frame interval to acceptance of information to be transmitted to other transmitter-receivers of the system and for randomly selecting as an initial transmission information message interval one of said information message intervals from among available ones of said predetermined plurality of information message intervals in the initial transmission frame interval next following said current frame interval; random retransmission time selection means for randomly selecting as a retransmission information message interval one of said information message intervals from among said predetermined plurality of information message intervals in a retransmission frame interval following said initial transmission frame interval by at least said delay; announcing and transmission means coupled to said random initial transmission time selection means and to said random retransmission time selection means for transmitting an announcement message during the minislot interval of said initial transmission information message interval, and within said minislot interval of said initial transmission information message interval, within that minislot corresponding in minislot sequence within said minislot interval to the sequential position of said one of said information message intervals within said retransmission frame interval, thereby announcing to all said associated transmitter-receivers that particular information transmission interval during which retransmission of said information to be transmitted will occur in the event that a collision occurs in said initial transmission information message interval within said initial transmission frame interval; collision identifying means for identifying collisions within said information message intervals; and identification and inhibiting means coupled to said collision identifying means and to said random initial transmission time selection means for identifying said available ones of said predetermined plurality of information message interval in said retransmission frame interval, said inhibiting means excluding as available information message intervals those information message intervals received from said system which are both announced as being for retransmission of information in the event of a collision, and identified by said collision identifying means as being associated with a collision.
 6. A contention transmitter-receiver for a contention system for transmitting information among a plurality of similar transmitter-receivers by way of a transmission path having a time delay between any transmitter and any receiver which is long by comparision with the duration of an information message, the system including:timing means for establishing a uniform time frame among said transmitter-receivers, said time frame including recurrent frame intervals, each frame interval including a common minislot interval and an information interval including a predetermined plurality of message slots, said common minislot interval including a plurality equal to said predetermined plurality of time-sequential common minislots, said message slots each including an information message interval and an individual minislot interval including a plurality equal to said predetermined plurality of individual minislots, said transmitter-receiver comprising: random initial transmission time selection means for accepting during a current frame interval information to be transmitted and for selecting as an initial transmission message slot one of a plurality of available ones of said predetermined plurality of message slots in an initial transmission frame interval following said current frame interval; random retransmission time selection means for randomly selecting as a first retransmission message slot one of said message slots in a first retransmission frame following said initial transmission frame by at least said delay; announcing and transmission means responsive to said random initial transmission time selection means and to said random retransmission time selection means for transmitting said information during said initial transmission message slot, and for, during said individual minislot interval of said initial transmission message slot, transmitting a first retransmission announcement in that individual minislot corresponding in individual minislot sequence with said individual minislot interval to the sequence position of said first retransmission message slot within the sequence of said message slots of said first retransmission frame, thereby announcing to all said transmitter-receivers that particular message slot in which retransmission of said information is scheduled to take place in the event of a collision occurring in said initial transmission message slot; collision identification means for identifying a collision within said information message interval, within said individual minislot interval of said initial transmission message slot and within said common minislot interval of said initial transmission frame interval; counting means coupled to said collision identification means for establishing the number of collisions which occur in each frame for which each retransmission message slot is selected, and for identifying said first retransmission message slot as one of (a) having been selected once in said first retransmission frame and therefore being a valid first retransmission slot and (b) having been selected more than once in said first retransmission frame and therefore being invalid; first inhibiting means coupled to said counting means and to said random initial transmission time selection means for excluding from said plurality of available message slots those message slots of a frame interval designated as valid; random second retransmission time selection means coupled to said counting means, for, in response to designation of said first retransmission message slot as invalid, randomly selecting a further retransmission message slot from among message slots of a further retransmission frame following said first retransmission frame by at least said delay; and first retransmission means coupled to said random retransmission time selection means, to said random second retransmission time selection means, and to said counting means for retransmitting said information during said first retransmission message slot if said counting means designates it as valid, and, when said counting means designates said first retransmission message slot as invalid, for inhibiting retransmission of said information during said first retransmission frame, and for, during said common minislot interval of said first retransmission frame, transmitting a further retransmission announcement in that common minislot corresponding in common minislot sequence to the sequence position of said further retransmission message slot within the sequence of said message slots of said further retransmission frame, thereby announcing to all said transmitter-recievers that Particular message slot in which further retransmission of said information is scheduled to take place.
 7. A method for communicating information among a plurality of transmitter-recievers by way of a transmission path having a path time delay which is long by comparison with the duration of a packet of information, comprising the steps of:establishing a uniform time frame among said transmitter-receivers, including recurrent frame intervals; establishing within each of said frame intervals a predetermined plurality of time-sequential message slots; esabilishing within each of said message slots a time sequence of an information interval and a second plurality of time sequential minislots, said second plurality being equal to said first plurality; at a first transmitter-receiver of said plurality of transmitter-receivers, accepting during a current frame interval first information to be transmitted to all of said transmitter-receivers; at said first transmitter-receiver, organizing said first information into a first original packet to be transmitted over said transmission path; at said first transmitter-receiver, randomly selecting a first original transmission message slot from among said first plurality of message slots in a transmission frame interval next following said current frame interval, said first original transmission message slot being selected for original transmission of a first original information packet including said first information; at said first transmitter-receiver, randomly selecting a first retransmission message slot from among said plurality of message slots in a retransmission frame interval following said transmission frame interval by a period at least equal to said path time delay; transmitting into said transmission path at said first transmitter-receiver said first original packet during said information interval of said first original transmission message slot and a first retransmission signal during that minislot corresponding in minislot sequence position to the sequential position of said first retransmission message slot within said plurality of message slots in said retransmission frame; at a second transmitter-receiver of said plurality of transmitter-receivers, accepting during said current frame interval second information to be transmitted to all of said transmitter-receivers; at said second transmitter-receiver, organizing said second information into a second orginal packet to be transmitted over said transmission path; at said second transmitter-receiver, randomly selecting a second original transmission message slot from among said first plurality of message slots in said transmission frame interval, said second original transmission message slot being selected for original transmission of a second original information packet including said second information; at said second transmitter-receiver, randomly selecting a second retransmission message slot from among said plurality of message slots in said retransmission frame interval; transmitting into said transmission path at said second transmitter-receiver said second original packet during said information interval of said second original transmission message slot and a second retransmission signal during that minislot corresponding in minislot sequence position to the sequential position of said second retransmission mesage slot within said plurality of message slots in said retransmission frame, whereby if said first and second original transmission message slots happen to be identical a collision occurs and said first and second information is scrambled as received, but said first and second retransmission signals are identifiable; at all said transmitter-receivers, generating a collision signal in response to an inability to recover information during said collision; at all said transmitter-receivers except said first and second transmitter-receivers, inhibiting transmission during said first and second retransmission message slots of said retransmission frame interval in response to said collision signal; at said first transmitter-receiver, in response to said collision signal, retransmitting said first information as a first retransmission packet during said first retransmission message slot in said retransmission frame; at said second transmitter-receiver, in response to said collision signal, retransmitting said second information as a second retransmission packet during said second retransmission message also in said retransmission frame. 